Typically, a motor controller consists of an inverter circuit. FIG. 6 shows an example of this type of motor controller. A conventional motor controller 500 shown in FIG. 6 is partially disclosed in JP-A-2006-280091. The conventional motor controller 500 is configured to control a three-phase brushless direct current (DC) motor 1. For example, the motor 1 is used as a power source for a device that governs valve cam timing of an engine of a vehicle.
A three-phase inverter circuit 2 for driving the motor 1 includes six N-channel power metal oxide semiconductor field-effect transistors (MOSFETs) Q1-Q6. The MOSFETs Q1-Q6 are connected in a three-phase bridge configuration between a power supply terminal +V and a ground terminal. The MOSFETs Q1, Q3, Q5 form an upper side arm of the inverter circuit 2. The MOSFETs Q2, Q4, Q6 form a lower side arm of the inverter circuit 2. The MOSFETs Q1, Q2 are connected to each other at a node connected to an U-phase coil of the motor 1. The MOSFETs Q3, Q4 are connected to each other at a node connected to a V-phase coil of the motor 1. The MOSFETs Q5, Q6 are connected to each other at a node connected to a W-phase coil of the motor 1. As can been seen from FIG. 6, each of the MOSFETs Q1-Q6 is provided with a flywheel diode. A pi-type LC filter circuit 3 is connected between the power supply terminal +V and the ground terminal.
Three Hall effective integrated circuits (ICs) 1u, 1v, 1w detect a rotation position (rotor position) of the motor 1 and output position detection signals Su, Sv, Sw, respectively, to a control circuit 4. The position detection signals Su, Sv, Sw are phase-shifted to each other by 120 degrees. The control circuit 4 detects a rotation direction of the motor 1 based on the position detection signals Su, Sv, Sw and outputs a rotation speed signal VN having a voltage level proportional to an actual rotation speed of the motor 1. The control circuit 4 has a frequency to voltage (F/V) converter 12 that converts a rotation frequency corresponding to the actual rotation speed of the motor 1 into the rotation speed signal VN. The F/V converter 12 is not disclosed in the drawings of JP-A-2006-280091.
The control circuit 4 outputs gate control signals to the inverter circuit 2 based on the rotation direction of the motor 1 and input timings of the position detection signals Su, Sv, Sw. The gate control signals are applied to gate terminals of the MOSFETs Q1-Q6, respectively. When the gate control signals are set to a first pattern, the motor 1 is driven to rotate in forward direction. In contrast, when the gate control signals are set to a second pattern, the motor 1 is driven to rotate in reverse direction. The gate control signals are directly applied to the gates of the MOSFETs Q1, Q3, Q5, respectively. In contrast, the gate control signals are applied to the gates of the MOSFETs Q2, Q4, Q6 through an AND gate 5, respectively. Although FIG. 6 illustrates one AND gate 5, the MOSFETs Q2, Q4, Q6 are individually provided with the AND gate 5.
The control circuit 4 has a rotational direction switch terminal T for receiving a rotation direction switch signal from a third comparison circuit 10. When the switch signal is at a low level, the gate control signals are set to the first pattern so that the motor 1 is driven to rotate in forward direction. In contrast, when the switch signal is at a high level, the gate control signals are set to the second pattern so that the motor 1 is driven to rotate in reverse direction.
An amplifier circuit 6 has a non-inverting input terminal for receiving a target voltage signal VS from a F/V converter 13 and an inverting input terminal for receiving the rotation speed signal VN from the control circuit 4. The F/V converter 13 converts a frequency signal, which is fed from an electronic control unit (ECU) of the vehicle, into the target voltage signal VS. The F/V converter 13 is not disclosed in the drawings of JP-A-2006-280091. The target voltage signal VS has a voltage level proportional to a target rotation speed of the motor 1. The amplifier circuit 6 generates a differential voltage signal Vo by amplifying a difference between the target voltage signal VS and the rotation speed signal VN by a predetermined gain G. The differential voltage signal Vo is given by: Vo=VS+G(VS−VN).
A first comparison circuit (i.e., comparator) 7 has a non-inverting input terminal for receiving the differential voltage signal Vo from the amplifier circuit 6 and an inverting input terminal for receiving a triangular wave signal VT from an oscillator circuit (not shown). For example, the triangular wave signal VT has amplitude of between 2.5 volts and 4 volts. The first comparison circuit 7 generates a first pulse-width modulation (PWM) signal P1 by comparing the differential voltage signal Vo with the triangular wave signal VT. The first PWM signal P1 is outputted to a signal switch circuit 8.
The signal switch circuit 8 switches between a forward rotation position and a reverse rotation position in accordance with a signal level of the switch signal outputted from the third comparison circuit 10. When the signal switch circuit 8 is in the forward rotation position, the first PWM signal P1 outputted from the first comparison circuit 7 is fed to a first input terminal of the AND gate 5. In contrast, when the signal switch circuit 8 is in the reverse rotation position, a second PWM signal P2 outputted from a second comparison circuit 9 is fed to the first input terminal of the AND gate 5. The gate control signals applied to the MOSFETs Q2, Q4, Q6 of the inverter circuit 2 are fed to a second input terminal of the AND gate 5.
The third comparison circuit 10 has hysteresis. The third comparison circuit 10 has an inverting input terminal for receiving the differential voltage signal Vo from the amplifier circuit 6 and a non-inverting input terminal for receiving a lower limit voltage VTL (i.e., 2.5 volts) of the triangular wave signal VT from a voltage signal generation circuit (not shown) such as a peak hold circuit. The third comparison circuit 10 generates the switch signal by comparing the differential voltage signal Vo with the lower limit voltage VTL. The switch signal is outputted to each of the control circuit 4 and the signal switch circuit 8. The signal level of the switch signal depends on the result of the comparison. Specifically, when the voltage level of the differential voltage signal Vo is equal to or greater than the lower limit voltage VTL (i.e., Vo≧VTL), the switch signal becomes the low level. In contrast, when the voltage level of the differential voltage signal Vo is less than the lower limit voltage VTL (i.e., Vo<VTL), the switch signal becomes the high level. In practice, the hysteresis of the third comparison circuit 10 affects the signal level of the switch signal.
As described above, the signal switch circuit 8 switches between the forward rotation position and the reverse rotation position in accordance with the signal level of the switch signal. The signal switch circuit 8 switches to the forward rotation position, when the switch signal is at the low level, i.e., when the voltage level of the differential voltage signal Vo is equal to or greater than the lower limit voltage VTL (i.e., Vo≧VTL). In contrast, the signal switch circuit 8 switches to the reverse rotation position, when the switch signal is at the high level, i.e., when the voltage level of the differential voltage signal Vo is less than the lower limit voltage VTL (i.e., Vo<VTL).
An amplifier circuit 11 with a gain of 1 has a non-inverting input terminal for receiving the differential voltage signal Vo from the amplifier circuit 6 and an inverting input terminal for receiving the lower limit voltage VTL. The amplifier circuit 11 outputs a correction voltage signal Vo2 by inverting the differential voltage signal Vo with respect to the lower limit voltage VTL. Specifically, the correction voltage signal Vo2 is given by subtracting the differential voltage signal Vo from 5 volts (i.e., Vo2=5−Vo).
The second comparison circuit 9 has a non-inverting input terminal for receiving the correction voltage signal Vo2 from the amplifier circuit 11 and an inverting input terminal for receiving the triangular wave signal VT. The second comparison circuit 9 generates the second PWM signal P2 by comparing the correction voltage signal Vo2 with the triangular wave signal VT. The second PWM signal P2 is outputted to the signal switch circuit 8.
According to the conventional motor controller 500, when the differential voltage signal Vo is equal to or greater than 2.5 volts (i.e., lower limit voltage VTL of the triangular wave signal VT), the signal switch circuit 8 switches to the forward rotation position. Thus, the first PWM signal P1 is fed from the first comparison circuit 7 to the AND gate 5 through the signal switch circuit 8.
In this case, the correction voltage signal Vo2 becomes less than 2.5 volts, because Vo2=5−Vo. Therefore, whereas the first PWM signal P1 has enable pulse width, a duty ratio of the second PWM signal P2 is 0 percent. Further, since the switch signal outputted form the third comparison circuit 10 is at the low level, the control circuit 4 sets the gate control signals to the first pattern. Thus, the motor 1 is feedback-controlled and driven to rotate in forward direction at the target rotation speed corresponding to the target voltage signal VS.
Then, if the target voltage signal VS sharply decreases, the difference between the target voltage signal VS and the rotation speed signal VN transiently increases. As a result, the differential voltage signal Vo is temporally reduced to less than 2.5 volts so that the switch signal outputted from the third comparison circuit 10 becomes the high level. Therefore, the signal switch circuit 8 switches to the reverse rotation position, and the second PWM signal P2 is fed from the second comparison circuit 9 to the AND gate 5 through the signal switch circuit 8.
In this case, the correction voltage signal Vo2 exceeds 2.5 volts, because Vo2=5−Vo. Therefore, whereas a duty ratio of the first PWM signal P1 becomes 0 percent, the second PWM signal P2 has enable pulse width. Further, since the switch signal outputted from the third comparison circuit 10 is at the high level, the control circuit 4 sets the gate control signals to the second pattern. Thus, reverse toque (braking torque) is applied to the motor 1 so that the speed of the motor 1 sharply decreases.
Then, when the difference between the actual rotation speed of the motor 1 and the target rotation speed of the motor 1 is reduced below a predetermined value, the differential voltage signal Vo exceeds the lower limit voltage VTL of the triangular wave signal VT again. As a result, the switch signal outputted from the third comparison circuit 10 becomes the low level. Therefore, the signal switch circuit 8 switches to the forward rotation position so that the first PWM signal P1 having the enable pulse width is fed to the AND gate 5. Thus, the motor 1 returns to a normal condition and is controlled based on the first PWM signal P1.
As a result, relationships among the actual rotation speed of the motor 1, the target voltage signal VS, and the rotation speed signal VN may become as shown in FIG. 7A, which is disclosed in JP-A-2006-280091. As can be seen from FIG. 7A, when the target voltage signal VS is in a first range between 2.5 volts and 4.0 volts, the motor 1 is driven to rotate in forward direction. In contrast, when the target voltage signal VS is in a second range between 1.0 volts and 2.5 volts, the motor 1 is driven to rotate in reverse direction.
However, FIG. 7A shows ideal relationships among the actual rotation speed of the motor 1, the target voltage signal VS, and the rotation speed signal VN. In practice, as shown in FIG. 7B, an offset voltage is added to the target voltage signal VS to reverse the rotation direction of the motor 1. If the offset voltage is not added to the target voltage signal VS, torque required to drive the motor 1 that is in a stopped state cannot be produced. The addition of the offset voltage to the target voltage signal VS is performed in the F/V converter 13 at a timing when the signal level of the switch signal changes.
The difference between the target voltage signal VS and the rotation speed signal VN temporally increases, when the rotation direction of the motor 1 is reversed. Further since the offset voltage is added to the target voltage signal VS, the difference increases by the offset voltage. Furthermore, the difference is amplified by the amplifier circuit 6 so that the differential voltage signal Vo outputted from the amplifier circuit 6 becomes larger. Accordingly, the duty ratio of the first and second PWM signals P1, P2 significantly increases. As a result, an electric current flowing through the motor 1 increases, and MOSFETs Q1-Q6 may be overheated. Therefore, excessive current occurs in the inverter circuit 2, and the MOSFETs Q1-Q6 may be thermally broken.